Radiogenic nuclides (more commonly referred to as radiogenic isotopes) form some of the most important tools in geology. They are used in two principal ways:

In comparison with the quantity of the radioactive 'parent isotope' in a system, the quantity of the radiogenic 'daughter product' is used as a radiometric dating tool (e.g. uranium-lead geochronology).

In comparison with the quantity of a non-radiogenic isotope of the same element, the quantity of the radiogenic isotope is used to define its isotopic signature (e.g. 206Pb/204Pb). This technique is discussed in more detail under the heading isotope geochemistry.

Contents

Some naturally-occurring isotopes are entirely radiogenic, but all these are isotopes that are radioactive, with half-lives too short to occur primordially. Thus, they are only present as radiogenic daughters of either ongoing decay processes, or else cosmogenic (cosmic ray induced) processes that produce them in nature freshly. A few others are naturally produced by nucleogenic processes (natural nuclear reactions of other types, such as neutron absorption).

For radiogenic isotopes that decay slowly enough, or that are stable isotopes, a primordial fraction is always present, since all sufficiently long-lived and stable isotopes do in fact naturally occur primordially. An additional fraction of some of these isotopes may also occur radiogenically.

Lead is perhaps the best example of a partly radiogenic substance, as all four of its stable isotopes (204Pb, 206Pb, 207Pb, and 208Pb) are present primordially, in known and fixed ratios. However, 204Pb is only present primordially, while the other three isotopes may also occur as radiogenic decay products of uranium and thorium. Specifically, 206Pb is formed from 238U, 207Pb from 235U, and 208Pb from 232Th. In rocks that contain uranium and thorium, the excess amounts of the three heavier lead isotopes allows the rocks to be "dated," or the time estimate from when the rock solidified and the mineral held the ratio of isotopes fixed and in place.

Other notable nuclides that are partly radiogenic are argon-40, formed from radioactive potassium, and nitrogen-14, which is formed by the decay of carbon-14.

Other important examples of radiogenic elements are radon and helium, both of which form during the decay of heavier elements in bedrock. Radon is entirely radiogenic, since it has too short a half-life to occur primordially. Helium, however, occurs in the crust of the Earth primordially, since both helium-3 and helium-4 are stable, and small amounts were trapped in the crust of the Earth as it formed. Helium-3 is almost entirely primordial (a small amount is formed by natural nuclear reactions in the crust). The global supply of helium (which occurs in gas wells and well as the atmosphere) is almost entirely (about 90-99%) radiogenic, as shown by its factor of 10 to 100 times enrichment in radiogenic helium-4 relative to the primordial ratio of helium-4 to helium-3. This latter ratio is known from extraterrestrial sources, such as some moon rocks and meteorites, which are relatively free of parental sources for helium-3 and helium-4.

As noted in the case of lead-204, a radiogenic nuclide is often not radioactive. In this case, if its precursor nuclide exhibits a half life too short to survive from primordial times, then the parent nuclide will be gone, and known now entirely by a relative excess of its stable daughter. In practice, this occurs for all radionuclides with half lives less than about 50 to 100 million years. Such nuclides are formed in supernovas, but are known as extinct radionuclides, since they are not seen directly on the Earth today.

An example of an extinct radionuclide is xenon-129, a stable isotope of xenon which appears as a relative excess against other xenon isotopes. It is found in meteorites that condensed from the primordial solar system dust cloud and trapped promordial iodine-129 (half life 15.7 million years) some time in a relative short period (probably less than 20 million years) between the iodine-129's creation in a supernova, and the formation of the solar system by condensation of this dust. The trapped iodine-129 now appears as a relative excess of xenon-129. Iodine-129 was the first extinct radionuclide to be inferred, in 1960. Others are aluminium-26 (also inferred from extra magnesium-26 found in meteorites), and iron-60.

The following table lists some of the most important radiogenic isotope systems used in geology, in order of decreasing half-life of the radioactive parent isotope. The values given for half-life and decay constant are the current consensus values in the Isotope Geology community.[1] ** indicates ultimate decay product of a series.